This invention relates generally to a system (method and apparatus) for removing emissions from the exhaust gases of a vehicle""s internal combustion engine and more particularly to the reduction of nitrogen oxides to free nitrogen.
The invention is particularly applicable to and will be described with specific reference to a system for the removal of nitrogen oxides from diesel engines operating at lean air/fuel ratios which promote the formation of NOX. However, those skilled in the art will understand that the invention has broader application and could be applied, for example, to gasoline engines operated at lean burn conditions, or any engine exhaust stream containing oxygen.
The following documents are incorporated by reference so that details known to those skilled in the art may not have to be restated when explaining the invention. The following documents are not part of the present invention.
1) xe2x80x9cThermal Cracking of Higher Paraffinsxe2x80x9d by H. H. Voge and G. M. Good, Journal of American Chemical Society, Vol. 71, pages 593-597, February, 1949;
2) SAE paper No. 98 FL 577, xe2x80x9cPlasma-Assisted Catalytic Reduction of NOXxe2x80x9d by B. M. Penetrante, R. M. Brusasco, B. T. Merritt, W. J. Pitz, G. E. Vogtlin, K. E. Voss, C. Z. Wan, M. C. Kung, H. H. Kung, D. W. Kim, and P. W. Park, dated October 1998;
3) xe2x80x9cCatalytic reduction system of NOX in exhaust gases from diesel engines with secondary fuel injectionxe2x80x9d by T. Nakatsuji, R. Yasukawa, K. Tabata, K. Ueda, and M. Niwa, pages 333-345, Applied Catalysts B: Environmental Aug. 17, 1998, (copyright) Elsevier Science B.V.;
4) U.S. Pat. No. 5,440,876 to Bayliss et al., Aug. 15, 1995; U.S. Pat. No. 5,586,433 to Boegner et al., Dec. 24, 1996; U.S. Pat. No. 5,711,147 to Vogtlin et al., Jan. 27, 1998; and U.S. Pat. No. 5,788,936 to Subramanian et al., Aug. 4, 1998.
This invention is directed to the removal of nitrogen oxides, NOX, from the exhaust gases of internal combustion engines, particularly diesel engines, which operate at combustion conditions with air in excess of that required for stoichiometric combustion, i.e., lean. It is well known that fuel efficiency improvements in excess of 10% can be achieved in gasoline engines operated at xe2x80x9clean burnxe2x80x9d conditions when compared to today""s engines which cycle the air to fuel ratio about stoichiometric. Diesel engines, by their nature, operate at lean conditions and have always achieved high fuel efficiencies.
There are numerous ways known in the art to remove NOX from a waste gas. This invention is directed to a catalytic reduction method for removing NOX. A catalytic reduction method essentially comprises passing the exhaust gas over a catalyst bed in the presence of a reducing gas to convert NOX into nitrogen. Two types of catalytic reduction are practiced. The first type is non-selective catalyst reduction and the second type is selective catalyst reduction (SCR). This invention relates to SCR systems.
In the selective catalyst reduction method, a reducing agent or reductant is supplied to the exhaust stream and the mixture is then contacted with a catalyst. Typical reducing agents used in industrial processes for removal of NOX from waste streams, such as urea or ammonia, are not suited for vehicular applications because of the large quantities of the reducing agent required. Any SCR method using a separate reducing agent is simply not practical for vehicle applications. Additional storage tanks have to be provided. Also, measures have to be taken to account for the environmental affects on the reducing agent such as freezing. In addition, space limitations for the reactor have to be addressed.
The prior art has recognized the limitations present in supplying a separate reducing agent for converting NOX and has used diesel fuel itself as the reducing agent. In U.S. Pat. No. 5,343,702 to Miyajima et al., issued Sep. 6, 1994, excess fuel is injected into the combustion chambers of the engine on a sensed demand basis to provide unsaturated HC to the catalyst. In U.S. Pat. No. 5,189,876 to Hirota et al., issued Mar. 2, 1993, diesel fuel is injected into a cracking chamber formed in the exhaust manifold of the engine. In U.S. Pat. No. 5,788,936 to Subramanian et al., issued Aug. 4, 1998, a wide variety of reductants is introduced into the exhaust stream upstream of an acidic alumina catalyst which is said to be effective to reduce NO2 to N2. In U.S. Pat. No. 5,586,433 to Boegner et al., issued Dec. 24, 1996, a separate cracking unit for diesel fuel is supplied. The ""702 and ""876 patents simply inject diesel fuel into the system. Systems injecting diesel fuel, in liquid form, into the exhaust stream must include provisions for warming the fuel to overcome low temperature conditions encountered in a vehicular environment. In contrast, the ""433 patent separately cracks diesel fuel with air and will produce a more reactive reducing agent. A somewhat similar concept to the ""433 patent appears to be disclosed in PCT application WO 97/01697 published Jan. 16, 1997. Still further, the catalytic reduction article published by Elsevier Science, B.U., shows admission of fuel and air over a partial oxidation catalyst to form acetaldehyde and formaldehyde as a reductant.
Recently plasma techniques have been proposed for use in emission systems. The method of using plasma techniques can take various forms such as microwave, RF, and even glow discharge. However, many systems utilize corona discharge techniques to generate the plasma. Initially, it was believed that the plasma would reduce NOX to elemental nitrogen, N2. (See, for example, U.S. Pat. No. 5,746,984 to Hoard, issued May 5, 1998, using a non-thermal plasma to reduce NOX and U.S. Pat. No. 5,715,677 to Wallman et al., issued Feb. 10, 1998 using a thermal plasma. See also U.S. Pat. No. 5,440,876 to Bayliss et al., issued Aug. 15, 1995) However, as explained in U.S. Pat. No. 5,711,147 to Vogtlin, issued Jan. 27, 1998, the plasma converts NOX to NO2 which must then be subsequently reduced by a selective catalyst.
The NOX plasma art has also utilized a reducing agent with the plasma. In U.S. Pat. No. 5,807,466 to Wang et al., issued Sep. 15, 1998, diesel fuel is injected upstream of a NOX plasma reactor to produce N2. In U.S. Pat. No. 5,767,470 to Cha, issued Jun. 16, 1998, diesel fuel is injected into the exhaust gas prior to entering a two stage microwave converter. In the ""147 patent, diesel fuel is passed through a plasma prior to mixing with the exhaust gas and the entire mixture is passed through a second plasma reactor. See also European patent application 0 366 876 B1 published May 12, 1993, and the ""876 patent for injecting other reducing agents, i.e., ammonia, into the exhaust gas passing through the plasma.
All of these approaches pass the entire exhaust stream with or without a reducing agent through a plasma reactor. Energy is required to generate the plasma and because the entire exhaust stream is treated by the plasma, a significant amount of energy must be developed by the vehicle to produce the plasma. Early plasma units placed an electrical load on the vehicle approximately equal to a 10% fuel penalty. Further, the load is a constant horsepower drain. Thus, with the vehicle cruising or idling at minimal engine fuel usage, the drain to operate the reactor represents a higher fuel utilization than the 10% value. It makes little sense to operate the engine at a lean burn condition to produce an overall fuel savings of about 10% if that savings is used by the energy required to power a reactor to eliminate NOX emissions which are in significant quantities because the engine is operated lean. Recent developments in the reactors have led to published literature claims of less energy utilization. The fact remains that the entire exhaust gas stream must be passed through the plasma which must be sufficiently energetic to induce reactions with the gases passing through the plasma. The prior art plasma NOX reduction systems are inherently energy inefficient systems.
Apart from using the plasma to react with NOX in a manner which converts the NOX or allows further conversion of the NOX to N2, plasma has been generated in air to react with or oxidize coke inherently produced by diesel engines. The ""677 and the ""876 patents illustrate plasma reactors burning soot with air introduced in a metallic bed situated in a plasma field, i.e., thermal plasma or a ferro-electric bed in the plasma, i.e., non-thermal plasma.
It is thus a principal object of this invention to provide an energy efficient selective reduction catalyst system capable of converting NOX emissions into N2 when the engine is operated at lean air/fuel conditions by a highly reactive reducing gas and more specifically by the, utilization of a plasma to produce a highly reactive reducing gas.
This object along with other features of the invention is achieved in a system (method and apparatus) for converting noxious emissions in the exhaust stream produced by an internal combustion engine operating at lean air/fuel ratios into benign emissions which includes an arrangement that converts a portion of liquid engine fuel into a highly reactive mixture functioning as a gaseous reductant. The highly reactive mixture is introduced into the exhaust stream upstream of a reducing catalyst whereby a small quantity of fuel produced with little energy expenditure effectively disposes of oxides of nitrogen resulting from lean burn engine conditions.
In accordance with a particularly important feature of the invention, the reactive mixture is characterized as substantially comprising (i.e., more than 50%) one or more HC reductants selected from the group consisting of i) normal, unbranched aliphatics having a chain length where the number of carbon atoms per organic molecule is equal to or greater than 7, ii) olefins and iii) oxygenates. Branched aliphatic and aromatic organic molecules typically present in significant quantities when diesel fuel or fuel oil is injected into the exhaust stream are minimized.
In accordance with a more specific feature of the invention, a specific species of the reactive mixture can be generated having a significant quantity (which may be less than 50%) of short chain HC, such as oxygenated organic molecules whereby the reactive mixture is able to permeate conventional catalyst beds having microporous crystallite structures such as catalysts containing zeolites to significantly increase the contact area of the catalyst and improve the efficiency of the catalyst to reduce oxides of nitrogen to free nitrogen. The low weight or short chain oxygenated organic molecules having few carbon atoms (i.e. 3) would include at least one oxygen atom for every three carbon atoms to produce reactive reducing compounds.
In accordance with the broad inventive concept of the invention several arrangements can be used to produce the highly reactive mixture such as a conventional cracking unit producing short chain HC molecules which are further reacted with oxygen over an oxidation catalyst to produce the reactive mixture. However, an especially important feature of the invention is the utilization of a non-thermal plasma reactor which is particularly suited to generate the highly reactive mixture in the presence of oxygen in an effective and an energy efficient manner. In fact, the efficiencies of the plasma reactor for generating a reducing gas and its suitability for use in the harsh environment which the vehicle is exposed to, provide a stand alone inventive feature even if the plasma reactor was operated to simply produce a reducing agent which would parallel the reactivity of conventional reducing agents generated from the engine fuel and not the highly reactive reducing mixture discussed above.
In accordance with another aspect of the invention, the fuel is diesel fuel and a carrier gas having some oxygen content is admitted with the fuel into the non-thermal plasma reactor chamber. The oxygen has a high probability for disassociation within the plasma with the result that oxygen atoms are simultaneously incorporated into organic molecules produced by the dissociation of the fuel oil caused in good part by the plasma. The organic molecules thus produced are to a significant extent short chain, unbranched, reactive organic molecules which are generated in a most energy efficient manner.
In accordance with another feature of the invention, the non-thermal plasma reactor chamber includes spaced electrodes and a bed of dielectric particles between the spaced electrodes so that the reaction chamber is a dielectric barrier plasma reaction chamber with the plasma generated by an AC potential thus avoiding arc discharge occurrences otherwise requiring sophisticated electronic circuits sensing and controlling voltage pulsing to the reaction chamber.
In accordance with yet another feature of the invention, the carrier gas is either a portion of the exhaust gas (i.e., EGR) or air whereby coking within the pellet bed is avoided by the presence of oxygen in the carrier gas which can react with any free carbon over a large effective surface area resulting from the dielectric bed.
In accordance with another specific feature of the invention, the dielectric particles in the bed not only cause a uniform plasma throughout the bed but may be optionally coated with oxidation catalysts to speed the desired reactions.
In accordance with an important system feature of the invention, the plasma reactor can simply receive the oxygen and fuel oil to produce the reactive material as stated above or optionally, a partial oxidation catalyst may be provided upstream of the plasma reactor to trim the reactive mixture to a desired composition or optionally a thermal cracking unit can be provided downstream of the plasma reactor such that a small sized plasma reactor with oxygen will further reduce the diesel fuel while forming the reactive gas material whereby a number of different systems with specific attributes can be configured utilizing the benefits obtained in a reducing gas produced by free radical oxidation reactions caused by the plasma reactor.
It is a general object of the invention to produce from diesel and like fuels, a highly reactive, reducing gas for use in an SCR catalyst system.
It is another object of this invention to provide a system, method and apparatus, for reducing NOX emissions created by diesel engines by using diesel fuel a) so that separate tanks do not have to provided on the vehicle for a reducing gas and/or b) a minimal amount of fuel is used to produce the reducing gas because of the highly reactive nature of the reducing gas produced by the invention.
It is yet another object of the invention to provide a system for cracking diesel fuel and/or long chain HC, by a plasma reactor to produce a highly reactive gas mixture.
An important general object of the invention is to provide a plasma reactor through which fuel and a carrier gas are passed to produce a reducing agent for an SCR catalyst system.
Yet another more specific object of the invention is the provision of a plasma reactor for cracking of diesel fuel in a plasma reactor which avoids coke.
Another specific but important object of the invention is a system (method and apparatus) which utilizes a plasma reactor to produce from diesel fuel a highly reactive reducing gas characterized by a short chain molecular structure able to permeate a catalyst bed to achieve high NOX reduction efficiencies thereby permitting less fuel to be used as a reducing gas when compared to other conventional arrangements using diesel fuel as a reducing gas in an SCR system.
A more specific object of the invention is the provision of an SCR system for removing noxious emissions produced by lean burn engines which uses a plasma reactor to produce a reducing agent having desired reactive organic molecular structures by generating strong energetic plasmas on a minor portion of the engine""s fuel without imposing excessive energy loads on the engine.
A more subtle but important object of the invention is to generate a highly reactive reducing gas for internal combustion engines operating at lean burn conditions whereby downstream NOX catalysts can be developed or existing catalysts can be utilized without the deactivation or thermal degradation concerns which must currently be addressed.
An important object of the invention is the utilization of a carrier gas with diesel fuel in a plasma reactor to produce a highly reactive reducing gas at lower temperatures than required for conventional thermal cracking of diesel fuel.
A more subtle but important object of the invention is to provide, from the engine""s fuel, a highly reactive reducing gas mixture for use in an SCR system which is able to maintain its reactivity notwithstanding the harsh vehicular environment, especially the cold temperature limits, which it is subjected to.
These objects and other features of the invention will become apparent to those skilled in the art from the following Detailed Description of the Invention taken together with the accompanying drawings.